1. Field of the Invention
The present invention pertains to a circuit arrangement comprised of a send path carrying a send signal; a receiving path carrying a received signal;, an adaptive filter; a compensator in the received path; a digital/analog converter; a transmission unit; and a transmission line attached to the transmission unit, wherein over the transmission line at least the send signal, the received signal and echo signal portions are transmitted, with the send path and the received path being connected with the transmission unit; with the send signal, for the estimation of the echo signal, being transmitted to the adaptive filter and wherein the estimated digital echo signal is transmitted to the compensator for the reduction of the echo signal portion in the received path.
2. Discussion of the Background of the Invention and Material Information
Copper lines or wiring continue to serve as the main transmission medium of the transfer of data between a subscriber connection and a communications office of a public communication network. A subscriber connection must, on one side, be economically advantageous for the subscriber, yet on the other hand, it is desirable to realize the highest possible data transmission rate. Via the replacement of copper wire with glass fiber cables, the noted second condition could be largely fulfilled, however, this would be coupled with substantially increased costs, since the existing detailed distribution net would have to be completely replaced. For this very reason, the development of a fast digital fully duplex data transmission line, via copper wires, has already been suggested repeatedly. An overview of this suggested data transmission denoted as HDSL (High Bit Rate Digital Subscriber Line) was, for example, published in an article by J. W. Lechleider, entitled "High Bit Rate Digital Subscriber Lines: A Review of HDSL Progress" published in the IEEE Journal on Sel. Area in Com., 9(6), Pages 769 to 784, August 1991.
With HDSL data transmission, data is still transmitted in both directions via a copper cable. This so-called transmission unit, also denominated as terminating unit, controls the directional separation of the signals to be sent and to be received. Since this directional separation, particularly due to the changing characteristics of the lines, cannot be perfectly realized, the signal received, by a distant subscriber, is superimposed with a echo signal (also denominated as error signal) of the nearby subscriber. The echo signal can be about 30 to 40 dB stronger than the actual useful signal. In order to achieve, at the receiver, the required bit error rate of about 10.sup.-7 an adaptive compensation of the echo is undertaken, in that, via the use of an adaptive filter, the echo signal is estimated and subtracted from the received signal. For such an adaptive filter, for example, FIR (Finite Impulse Response) filters are utilized, which can have 90 to 100 coefficients (Taps) and as result thereof require a correspondingly large basic circuit block. In order to reduce the circuit expenditure, European Patent Application EP-0 384 490 discloses an echo canceler with an adaptive filter that is comprised of FIR (Finite Impulse Response) and an IIR (Infinite Impulse Response) filter, in a tandem or cascade connection. Therewith, a reduction of the circuit expenditure for the adaptive filter is achieved, however with this adaptive filter non-linearities in the send path cannot be compensated.
In addition, European Patent Application EP-0 464 500 discloses an echo canceler in which the send path, in addition to the amplifier elements, includes at least one delay element, a digital/analog converter and a analog low-pass filter, wherein particularly the digital/analog converter, due to the required high resolution and the simultaneously required high linearity, can only be implemented with a correspondingly great expenditure.
Furthermore, in an article authored by R. Batruni et al. and entitled "Mixed Digital/Analog Signal-Processing for a Single-Chip 2B1Q U- Interface Transceiver" (IEEE Journal of Solid-State Circuits, Vol 25, No. 6, published December 1990, pages 1414 to 1425), an echo path is described which is comprised of a digital prefilter, a digital/analog converter and an analog smoothing filter. Therein, with the aid of the prefilter, the send signal is so preformed, that after its transformation into an analog send signal, with the aid of the digital/analog converter, it must only yet be filtered with an analog smoothing filter having low-pass filter characteristics. While an approximately optimum analog send signal is obtained via this circuit arrangement, such a digital prefilter and the necessary higher linear digital/analog converter, require relatively great circuit expenditures.
The present invention thus has the task or object to produce a circuit arrangement for the production of an analog send signal wherein the circuit expenditures are significantly reduced.